Scenario: A pipe is attached to a speaker which is causing to vibrate, and at the same time a strobe light in a dark room is flashing in sync with the vibrations.
Why does the stream of water appear to zigzag?
The waves produced in the water have the same frequency of the sound produced by the speaker. The strobe emits light with a frequency that can be regulated by the user. Since the room is dark, you can only see the movement of the waves in the water in the moments when it is illuminated by strobe light.
In the case where the speaker emits sound at a fixed frequency that matches the frequency of the stroboscope, waves in water will have the appearance that are stationary; this means that they look like a snapshot of the wave. This is because the light illuminates the water ripples whenever they have the same shape.
On the other hand, if the speaker emits sound with variable frequency, such as the sound of music, and the strobe emits light with a fixed frequency, the effect we see in the waves of water is as if they were moving in zigzac because the strobe light illuminates the water ripples, in moments when the waveform is different.